The present invention relates generally to medical imaging, and more particularly to selectively attenuating a stream of radiation to which a patient is exposed. Specifically, the present technique relates the use of a configurable mask to optimize the X-ray flux incident on a patient such that the best image quality per unit dose of radiation is achieved for the target area.
In X-ray imaging systems, radiation from a source is directed toward a subject, typically a patient in a medical diagnostic application. A portion of the radiation passes through the patient and impacts a detector. In digital X-ray imaging, the surface of the detector converts the radiation to light photons which are sensed. The detector is divided into a matrix of discrete picture elements or pixels, and encodes output signals based upon the quantity or intensity of the radiation impacting each pixel region. Because the radiation intensity is altered as the radiation passes through the patient, the images reconstructed based upon the output signals provide a projection of the patient's tissues similar to those available through conventional photographic film techniques.
Digital X-ray imaging systems are particularly useful due to their ability to collect digital data which can be reconstructed into the images required by radiologists and diagnosing physicians, and stored digitally or archived until needed. In conventional film-based radiography techniques, actual films are prepared, exposed, developed and stored for use by the radiologist. While the films provide an excellent diagnostic tool, particularly due to their ability to capture significant anatomical detail, they are inherently difficult to transmit between locations, such as from an imaging facility or department to various physician locations. The digital data produced by direct digital X-ray systems, on the other hand, can be processed and enhanced, stored, transmitted via networks, and used to reconstruct images which can be displayed on monitors and other soft copy displays at any desired location. Similar advantages are offered by digitizing systems which convert conventional radiographic images from film to digital data.
One of the issues which arises in X-ray imaging, as well as other medical procedures in which a patient is selectively exposed to radiation, is delivering the appropriate amount of radiation to the target tissue needed to produce the desired image while minimizing the radiation dose to the target tissue, but also non-target tissues and even non-patients, such as medical staff. In particular, non-target tissue near the target tissue may be unnecessarily exposed to the radiation stream. Likewise, the target tissue need only be exposed to the minimum dose of radiation necessary to produce images of the desired quality. Typically, this quality can be described in terms of a signal-to-noise ratio which increases as the square root of the X-ray dose, i.e., doubling the signal-to-noise ratio requires quadrupling the X-ray dose.
Some dose reduction may be accomplished by optimizing the energy spectrum produced by the X-ray tube. This is done by adjusting the accelerating voltage applied to the tube or by introducing a spectral filter between the X-ray tube and the patient. Both of these methods allow the spectral profile of the radiation reaching the patient to be modified.
More generally, X-ray exposure can be regulated by exposure management or by using information extracted from previous exposures. In other words, the patient is protected by limiting the number of exposure events to which he or she is exposed. Alternatively, the field-of-view, or area of irradiation, may be collimated to a reduced area which still allows imaging of the target tissue. This collimation, however, is of limited effectiveness as the system operator is typically limited to an assortment of collimators of fixed size and shape from which the operator chooses the “best fit”. Only rarely, will a prepared collimator of precisely the right dimensions be available.
In addition, the detector itself is typically sensitive to high radiation flux levels and may be damaged or experience degraded performance at such levels. In particular, the detector may become saturated at flux levels outside the desired dynamic range, degrading imaging system performance. Such high flux levels may result on the detector when the tissue thickness or X-ray attenuation is small or in areas where the radiation from the X-ray source is not attenuated before reaching the detector (e.g. peripheral areas). Collimators or attenuating filters, typically either plates or fluid filled bags, may be employed between the X-ray tube and the detector to reduce saturation or other flux-related detector problems. The collimators or filters are typically of fixed dimension and shape and are manually adjusted and positioned with varying degrees of accuracy. In addition, the fixed shapes of these devices do not generally match the complex and unique shapes of patient anatomy. There is a need, therefore, for improved spatial X-ray filtering, attenuating and collimating approaches that can provide more flexible and precise control of radiation delivery to areas of a patient or other target.